Chain and sprocket systems are often used in automotive engine systems to transmit rotational forces between shafts. For example, a sprocket on a driven shaft may be connected via a chain to a sprocket on an idler shaft. In such a chain and sprocket system, rotation of the driven shaft and driven sprocket will cause the rotation of the idler shaft and idler sprocket via the chain. In an automotive engine system, sprockets on the crankshaft may be used to drive one or more cam shaft sprockets.
The chains used in chain and sprocket systems typically comprise a plurality of link plates connected with pins or rollers or chains with the plurality of link plates having engagement teeth connected with pins and/or links. The sprockets typically comprise a circular plate having a plurality of teeth disposed around the circumference thereof. Located between adjacent teeth are roots having generally arcuate or semi-circular profiles for receiving the pins, rollers, or teeth of the chain. Each root has a root radius which is the distance from the center of the sprocket to a point along the root closest to the center of the sprocket. The sprocket roots and/or teeth are also associated with a pitch radius, which is the distance from the center of the sprocket to a pin axis which is part of a chain joint when the chain is seated on the sprocket.
In a conventional (“straight”) sprocket, the root radii are all substantially equal, and the sprocket's pitch radii also are substantially equal. However, it has been found that as a chain rotates around a straight sprocket, audible sound frequencies creating undesirable noise are often generated as the chain teeth, pins or rollers connecting the links of the chain contact the sprocket teeth and impact sprocket engagement surfaces or the roots disposed between adjacent teeth of the sprockets.
Sound frequencies and volume of such noise created by the operation of chain and sprocket systems typically vary depending on the chain and sprocket designs, chain rotational speed, and other sound or noise sources in the operating environment. In the design of chain and sprocket systems, it can be desirable to reduce the noise levels generated as the rollers, pins or teeth of a chain engage a sprocket.
In chain tension measurements, certain chain tensions originating from occurrences outside the chain and/or sprocket in a particular system may vary on a periodic or repeating basis, which often can be correlated to tension inducing events. For example, in automotive timing chain systems, it has been observed from chain tension measurements that the engagement and disengagement of each sprocket tooth or root with the chain often results in repeating tension changes. These chain tension changes may be correlated with potentially tension-inducing events, such as the firing of piston cylinders. Reducing these tensions and forces on chains may be of particular importance if the chains include elements where they do not have the properties of steel, such as ceramic elements as described in U.S. application Ser. No. 10/379,669.
The number of tension events that occur relative to a reference time period, as well as the amount of the tension change for each event may be observed. For example, in an automotive timing chain system, one may observe the number or frequency of tension changes in the chain relative to rotations of a sprocket or a crankshaft, as well as the magnitude of the tension change in the chain. A tensioning event that occurs once per shaft or sprocket rotation is considered a “first” order event, and an event occurring four times for each shaft or sprocket rotation is considered a “fourth” order event. Depending on the system and the relative reference period, i.e., rotations of the crankshaft or the sprocket (or another reference), there may be multiple “orders” of events in a crankshaft or sprocket rotation in such a system that originate from one or more tension sources outside the chain and sprocket. Similarly, a particular order of the sprocket rotation may include or reflect the cumulative effect of more than one tensioning event. As used herein, such orders of tensioning events occurring during a sprocket (or crankshaft) rotation also may be referred to as the orders of the sprocket (or crankshaft) or the sprocket orders (or crankshaft orders).
In straight sprockets, measurable tensions typically are imparted to the chain at a sprocket order corresponding to the number of teeth on the sprocket, also known as the pitch order. Thus, in a sprocket with nineteen teeth, tensions would be imparted to the chain at the nineteenth order, i.e., nineteen times per revolution of the sprocket. This is engagement order. A tension event in a straight sprocket originating from outside the sprocket would typically occur at equal intervals relative to the sprocket rotation, with a generally equal tension change or amplitude.
A “random” sprocket typically has root and/or pitch radii that vary around the sprocket, i.e. it is not a straight sprocket. Random sprockets, in contrast, typically have different tensioning characteristics when compared to straight sprockets due to their differing root or pitch radii. As the chain rotates around the random sprocket, each of the different radii typically imparts a different tensioning event to the chain. For instance, as a roller of a roller chain engages a root having a first root radius, the chain may be imparted with a tension different from when a roller of the chain engages a root having a second root radius larger than the first root radius. Tension changes, in addition, may also be imparted to the chain by a random sprocket due to the relative positioning of the different root radii. A roller moving between adjacent roots having the same root radii may result in different chain tension changes than a roller moving between adjacent roots having different radii.
The change in chain tensions imparted by random sprockets due to the relative positioning of the root and/or pitch radii may be further accentuated when the sprocket has more than two different root or pitch radii. For example, in a random sprocket having first, second, and third successively larger root radii, the tension imparted to the chain may be greater when a chain roller moves from a root having a first root radii to a root having a third root radii than when a chain roller moves from a root having a first root radii to a root having a second root radii.
Random sprockets designed principally for noise reduction often cause increases in chain tensions and tension changes as compared to the maximum tensions imparted to the chain by straight sprockets. For example, a random sprocket design may reduce chain noise or chain whine by reducing the pitch order of the sprocket. However, reducing the pitch order of a sprocket may result in concentrating the tensional forces imparted to the chain by the sprocket over the lower orders of the sprocket. These lower orders can excite a chain drive resonance. This often results in increased chain tensions corresponding to the lower orders of the random sprocket.
Such increased chain tensions at the lower sprocket orders frequently cause the overall maximum chain tension force exerted on the chain and sprocket to increase. As a consequence, a chain and sprocket system subjected to such tensions typically will experience greater wear and increased opportunities for failure, as well as other adverse effects, due to the concentration of the tensional forces in the lower orders.
A recently issued U.S. Pat. No. 7,125,356 to Todd entitled “TENSION-REDUCING RANDOM SPROCKET” describes one approach for reducing chain tensions using repeating root and/or pitch radii patterns. The patent describes patterns or sequences effective to impart tensions to the chain to reduce maximum chain tensions during operation of the system relative to maximum chain tensions of a system. The disclosure of U.S. Pat. No. 7,125,356 to Todd is incorporated herein as if completely rewritten into this disclosure.
Generally speaking, when chain tensions reach a maximum or “spike” in a chain and sprocket system in an engine, a resonance condition has been reached and this resonance condition or mode typically corresponds to a given system oscillation frequency. As can be seen from U.S. Pat. No. 7,123,356, tension reducing sprockets providing tension reducing pitch radii or root radii patterns around the sprocket may be utilized to reduce such maximum tensions at resonance conditions.
In some systems, there may be more than one resonance mode, and a corresponding tension “spike”, though a range of system oscillation frequencies. Such multiple resonance conditions or modes may occur where there are multiple tensioning events from outside the chain and sprocket system interacting at one or more system frequencies; there are multiple chains and sprockets forming the system; or there are other excitations imposed on the system that results in multiple resonance modes. The presence of multiple resonance modes may complicate efforts to reduce overall chain tensions. For example, such a system having more than one driven sprocket may have a first resonance mode and a second resonance mode. A tension reducing sprocket designed and placed into the system, with a straight sprocket, to reduce tensions in the second resonance mode may increase chain tensions at the first resonance mode. Further, a similar result may occur if a second tension reducing sprocket is placed into the system to reduce the second (or other) resonance mode tensions.
While not intending to be bound by any theory, a chain and sprocket system may reach a resonance mode, with a tension spike, at relatively low frequency system oscillations. The chain and sprocket system may reach another (or more than one) resonance mode at higher system oscillation frequencies. The number of resonance modes and their corresponding system frequencies will depend on the nature and configuration of the systems, such as the chain stiffness, the number and types of sprockets used in the system, the sprocket and chain configuration, the nature and frequency of the external excitations imposed on the system, etc.
Again, without intending to be bound by theory, the system oscillations at the lower resonance modes tend to be in the same direction. At higher oscillation frequencies, part of the system will tend to oscillate in an opposite direction of another part of the system resulting in a second or other resonance mode. In some drive systems, the resonance mode at relatively high system oscillation frequencies does not contribute enough tension to the overall maximum chain tension to make it a concern for chain durability. In other drive systems, such as high inertia systems, the chain tensions at the resonance mode at such higher system oscillation frequencies can be significant. Further reducing tensions at the resonance mode at such higher oscillation frequency or higher resonance mode tensions can lead to improved drive efficiency, especially if a tension reducing sprocket has already been applied to reduce maximum tensions at first mode.